In electronics, a linear regulator is a device used to maintain a steady voltage. The resistance of the regulator varies in accordance with the load resulting in a constant output voltage. The regulating device is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage, and continually dissipating the difference between the input and regulated voltages as heat. Linear regulators may place the regulating device between the source and the regulated load (a series regulator), or may place the regulating device in parallel with the load (shunt regulator). Simple linear regulators may only contain a Zener diode and a series resistor whereas more complicated regulators include separate stages of a voltage reference, an error amplifier and a power pass element. Since linear voltage regulators are a common element of many devices, integrated circuit regulators are very common.
A pass transistor (or other device) is used as one half of a potential divider to establish the regulated output voltage for the linear regulator. The output voltage is compared to a reference voltage to produce a control signal to the transistor which will drive its gate or base. With negative feedback and suitable compensation selection, the output voltage is kept reasonably constant. Linear regulators are often inefficient since the pass transistor acting like a resistor wastes electrical energy by converting it to heat. The same regulating function can often be performed more efficiently by a switched-mode power supply, however a linear regulator may be preferred for light loads or where the desired output voltage approaches the source voltage. In these cases, the linear regulator may dissipate less power than a switching regulator. The linear regulator also has the advantage of not requiring magnetic devices (inductors or transformers) which can be relatively expensive or bulky.
One issue with linear regulator circuits is how they respond to line and/or load transients. One common technique is to monitor the output voltage of the regulator with a comparator. If a transient is sensed at the output by the comparator, an active device on the output of the regulator is turned on to sink current from the output and reduce the magnitude of the transient. Such output sinking techniques require a large die area for the active device to ensure the device can handle the power required during transient conditions. Such devices increase the size and cost of integrated circuit regulators which is an undesirable design goal for the regulator.